Conventionally, there is known a ceramic glow plug having a ceramic heater (hereinafter, may be referred to as a heater) which uses electrically insulating ceramic as a substrate and in which a resistance heat-generating element formed of a resistor or electrically conductive ceramic is embedded in the substrate or is provided on the outer surface (particularly, on the outer surface of a portion located toward the forward end) of the substrate. Even in use at such a high temperature that, for example, the surface temperature of the forward end of the heater becomes 1,350° C., in contrast to a metal glow plug having a resistance wire coil, the ceramic glow plug has low risk of occurrence of disconnection. Thus, conceivably, the ceramic glow plug is used while heat is generated at a relatively high temperature. Such a ceramic glow plug is configured such that the heater is held, at its outer circumference, by a tubular holding sleeve made of metal. The holding sleeve holds the heater and also establishes electrical connection through connection with an electrode lead-out portion of the heater adapted to supply electricity to a resistance heat-generating element of the heater (refer to, for example, Patent Document 1).
In recent years, emphasis has been placed on accurate grasping of the condition of combustion in order to control combustion with subtlety for the purpose of improving fuel economy and cleaning exhaust gas. In order to utilize a glow plug having a heater exposed to a combustion chamber for observing variation in pressure in the combustion chamber, there is provided a glow plug equipped with a pressure sensor having a built-in combustion pressure sensor (refer to, for example, Patent Document 2). In order to allow the heater exposed to the combustion chamber to move rearward in relation to a housing as a result of reception of combustion pressure, such a glow plug equipped with a pressure sensor has a movable member in the form of bellows or a diaphragm which serves as a seal and elastically holds the heater for allowing movement of the heater. The movable member is configured to be able to establish airtightness in order to prevent entry of combustion gas into the glow plug from the combustion chamber. Furthermore, in order to allow relative movement of the heater according to variation in combustion pressure, the movable member is formed by bending a steel sheet having a thickness of about 0.15 mm.